Here is some LSD info regarding change intervals:
http://www.camaro6.com/forums/showthread.php?t=436057
Here is the correct GM fluid for the LSD. Note it already contains friction modifiers.
http://www.gmpartsdirect.com/oe-gm/88862624
So, I think the question is to find out if they are using this exact product or some other 75w-90 gear oil (which may not contain modifiers).